1. Field of the Invention
The present invention relates to a swing type work machine such as a crane having a swing member provided with a boom or the like, as well as a method for setting a safe work area and a rated load according to a working state of the machine.
2. Description of the Related Art
Generally, in such a swing type work machine as above it is required, from the standpoint of safety, to prevent breakage and tipping during a swing work of the machine, and as means for satisfying such requirement it is very important to properly set a rated load and a safe work area, or a limit working radius, for operating the machine safely.
In the above rated load and safe work area there are included a strength-based rated load (safe work area) which is set taking the strength of each component into account and a stability-based rated load (safe work area) which is set taking the stability of the work machine into account. In determining the former, i.e., strength-based rated load (safe work area), importance is attached to the strength of a swing member such as a boom which becomes most disadvantageous in strength during a swing work, and a rated (safe work area) is established on the basis of the said strength. On the other hand, the latter, i.e., stability-based rated load (safe work area) is established for the purpose of preventing the tipping of the work machine during a swing work. Therefore, this rated load (safe work area) inevitably varies depending on the direction of the swing member such as a boom.
All of the above rated loads (safe work areas) are extremely important parameters in ensuring the safety of the work machine. According to the prior art, minimum values of the above strength-based rated load (safe work area) and stability-based rated load (safe work area), (more particularly, rated loads or safe work areas in a sideways protruded state of the boom in which the work machine is most likely to tip), are calculated and the smaller rated load (safe work area) is adopted as a safety parameter to be used actually, then a swing control or warning is performed in accordance with the thus-adopted rated load (safe work area).
In FIG. 13, strength-based safe work areas and stability-based safe work areas, which are calculated in an actual crane, are indicated by broken lines 91 and dash-double dot lines 92, respectively. More specifically, in a polar coordinate plane with a work radius and a wing angle as variables, strength-based safe work areas and stability-based safe areas, which correspond to specific hoisting loads, are shown in terms of contour lines.
In the same figure, O denotes a swing center of the swing member in the crane, FL denotes a support point by an outrigger jack protruded at the left front portion of the crane, FR denotes a support point by an outrigger jack protruded at the right front portion of the crane, RL denotes a support point by an outrigger jack protruded at the left rear portion of the crane, and RR denotes a support point by an outrigger jack at the right rear portion of the crane.
As noted above, since the strength-based safe work area is set taking the strength of the swing member of the boom or the like into account, its limit work radius is independent of the swing angle and the larger the hoisting load, the smaller the limit work radius. Therefore, the strength-based safe work areas corresponding to hoisting loads assume the shape of such concentric circles as shown by the broken lines 91 in FIG. 13.
On the other hand, the stability-based safe areas are set for preventing the tipping of the entire crane, so their schematic shapes describe a square contour line diagram surrounded with straight lines nearly parallel to tipping lines. Further, when a deformation of the boom is taken into account, there are described generally square shapes surrounded with curves which are centrally expanded somewhat outwards to an extent corresponding to the boom deflection rather than with straight lines parallel to tipping lines, as indicated by dash-double dot lines 92 in FIG. 13. The "tipping line" indicates a rotational center line at the time of tipping of the crane. For example, a tipping line in the left-hand side direction is a straight line connecting the support points FL and RL.
Thus, the stability-based safe work area originally assumes an irregular shape, so even at the same hoisting load, there ought to be different safe work areas or rated loads between the case where an article is hoisted sideways and the case where it is hoisted obliquely forward or obliquely backward. In a conventional crane or the like, however, a certain limit work radius, i.e., the smaller work radius between a minimum value of a limit work radius which depends on strength and a minimum value of a limit work radius which depends on stability, is established throughout the whole circumference, so the hoisting work particularly at an obliquely front position or an obliquely rear position is limited to a greater extent than necessary and hence the capacity thereof is not fully exhibited. This is also the case with setting rated loads.
In Japanese Patent Laid Open No.5,116889 ( a Japanese Patent Application corresponding to U.S. Pat. No. 5,217,126; hereby fully incorporated by reference) there is disclosed a device in which when outrigger jacks are protruded non-uniformly right and left, a safe work area is deformed into a shape other than a circle according to the protruded states. But this work area deformation takes into account only such non-uniform protrusion of outrigger jacks. Also in the said device, when all the outrigger jacks are protruded uniformly, certain limit work radium and rated load are set throughout the whole circumference. Thus, it cannot be said that the device disclosed in the above publication provides an effective measure for solving the foregoing problem.